Sivakoff et al. [1]} reported on another similar X-ray source in the elliptical galaxy NGC 4697 which showed two flares, and more recently,
Tiengo et al. [2]} reported a recurrent flaring activity from an ultra-luminous X-ray source in a globular cluster in the galaxy NGC 4472.
The properties of these sources are difficult to reconcile with those of better known Galactic accreting X-ray sources, such as the type I and II bursts observed in
some low-mass X-ray binaries and beamed emission from stellar mass black holes ([3]}; see, however, [1]}, [5]}). Therefore, it has
been proposed that they might constitute a new type of fast transients [3]}.
An explanation of observed X-ray bursts as originating from a tidal stripping of a white dwarf (WD) circulating an intermediate-mass black hole has been
proposed [7]}, [8]}, [9]}, [2]}.
Recently two other papers [11]}, [12]} reported data showing sequences of quasi-periodic highly energetic X-ray eruptions, with possible explanation that they might be driven by the disruption of a compact object orbiting a supermassive black hole [12]}, [14]}.
